It is known to electrically couple multiple switching sub-converters in parallel to increase switching power converter capacity and/or to improve switching power converter performance. One type switching power converter with multiple switching sub-converters is a “multi-phase” switching power converter, where the sub-converters switch out-of-phase with respect to each other. Such out-of-phase switching results in ripple current cancellation at the converter output filter and allows the multi-phase converter to have a better transient response than an otherwise similar single-phase converter.
As taught in U.S. Pat. No. 6,362,986 to Schultz et al., which is incorporated herein by reference, a multi-phase switching power converter's performance can be improved by magnetically coupling the energy storage inductors of two or more phases. Such magnetic coupling results in ripple current cancellation in the inductors and increases ripple switching frequency, thereby improving converter transient response, reducing input and output filtering requirements, and/or improving converter efficiency, relative to an otherwise identical converter without magnetically coupled inductors.
Two or more magnetically coupled inductors are often collectively referred to as a “coupled inductor” and have associated leakage inductance and magnetizing inductance values. Magnetizing inductance is associated with magnetic coupling between windings; thus, the larger the magnetizing inductance, the stronger the magnetic coupling between windings. Leakage inductance, on the other hand, is associated with energy storage. Thus, the larger the leakage inductance, the more energy stored in the inductor. Leakage inductance results from leakage magnetic flux, which is magnetic flux generated by current flowing through one winding of the inductor that is not coupled to the other windings of the inductor.
As taught in Schultz et al., large magnetizing inductance values are desirable to better realize the advantages of using a coupled inductor, instead of discrete inductors, in a switching power converter. Leakage inductance values, on the other hand, typically must be within a relatively small range of values. Leakage inductance must be sufficiently large to prevent excessive ripple current magnitude, but not so large that converter transient response suffers.
Coupled inductors which facilitate control of leakage inductance values have been proposed. For example, FIG. 1 shows a prior art two-winding or “two-phase” coupled inductor 100. Inductor 100 includes a magnetic core 102, which is shown as transparent. Magnetic core 102 forms a passageway 104, which is typically filled with non-magnetic material, such as air. First and second windings 106, 108 are wound through passageway 104. Current Io1 flowing through first winding 106 generates leakage magnetic flux following a path approximated by arrow 110, and current Io2 flowing through second winding 108 generates leakage magnetic flux following a path approximated by arrow 112. Leakage flux 110, 112 flows between windings 106, 108 and through passageway 104. Accordingly, leakage inductance is roughly proportional to separation 114 of windings 106, 108, and separation 114 therefore must be relatively large to achieve sufficiently large leakage inductance for typical applications.
As another example, United States Patent Application Publication Number 2009/0237197 to Ikriannikov et al., which is incorporated herein by reference, discloses, in part, coupled inductors including outer legs. The outer legs provide paths for leakage magnetic flux, and leakage inductance values can be adjusted, for example, by varying the configuration of gaps in the outer legs.
On the other hand, United States Patent Application Publication Number 2011/0032068 to Ikriannikov, which is incorporated herein by reference, discloses, in part, coupled inductors including a top magnetic element. The top magnetic element provides a path for leakage magnetic flux, and leakage inductance values can be adjusted, for example, by varying the configuration of the top magnetic element.